The present invention relates to semiconductor processing and, more particularly, to photolithographic processing of semiconductor materials. A major objective of the present invention is to improve the precision with which integrated circuit features can be defined using photolithography.
Much of modern technological progress is identified with miniaturization of integrated circuits. Miniaturization allows for increased functionality by increasing the number of circuits that can be integrated onto a single device. Increased processing speeds are also achieved as the capacitances and distances between circuit elements are reduced. In addition, miniaturization can lower costs by increasing the number of integrated circuit devices that can be made together.
Photolithography, a pervasive component of semiconductor processing, provides for the definition of integrated circuit features using patterns of light. Typically, a semiconductor wafer, at an initial or intermediate stage of processing, is coated with photoresist. A photoresist is a material whose solubility or reactivity changes as it is exposed to light. In photolithography, the light is patterned so that some regions of the photoresist are altered while others are not. A "negative" photoresist is one for which development removes unexposed photoresist, while a "positive" photoresist is one for which development removes exposed photoresist. Commonly, a positive photoresist is used which acidifies when exposed to ultraviolet light. An alkaline developer can be used to removed the exposed photoresist. The unexposed photoresist then masks the regions it covers, while the uncovered regions are subjected to subsequent processing steps.
Increasing miniaturization, i.e., increasing circuit density, requires that circuit features be made smaller and be arranged more closely together. However, small deviations in the manufacture of small, closely-spaced features can lead to non-functioning circuits. To increase the yield of integrated circuit manufacturing, circuits are made larger and spaced further apart than theoretically necessary so that tolerance is provided for errors in device placement and definition. Greater circuit density, as well as performance and economy could be achieved if photolithography could be made more precise.
The importance of photolithographic precision can be illustrated in the context of a metal-on-oxide (MOS) transistor. A MOS transistor functions as a switch having a source, a drain, and a gate. The source and drain can be electrically coupled and decoupled by varying the voltage applied to the gate. The gate of a MOS transistor lies over a channel in the semiconductor which has a conductivity type opposite to that of the source and drain regions. Conductivity type is determined by dopants introduced into the semiconductor material. If the dopant intended for the source and drain enters the intended channel region, the transistor will not function according to design. For this reason, the channel region is typically masked during introduction of the dopant intended for the source and drain.
The mask can be defined photolithographically. A layer of silicon dioxide can be formed over the semiconductor substrate. Positive photoresist can be spun on over the silicon dioxide. The photoresist can be exposed to ultraviolet light that is patterned so as to define the channel region. The exposed photoresist is dissolved and removed, while leaving unexposed photoresist over the source and drain. The portions of the silicon dioxide thereby uncovered are then chemically etched away, defining apertures through the silicon dioxide over the intended source and drain regions. The remaining photoresist can be removed at this point. The dopant for the source and drain is then introduced into the proper semiconductor regions. However, the dopant cannot penetrate through the silicon dioxide protecting the channel region.
One limitation on photolithographic precision is a tendency of the photoresist to curl away from the underlying surface. This can subject regions under the curl to unintended treatment. In the case of the MOS transistor, the chemical etchant can seep under the hardened photoresist and remove silicon dioxide from over the channel region. This can result in source/drain dopant entering the channel region, impairing or destroying the resulting transistor. This scenario can be accommodated by providing dimensional tolerances at the expense of circuit density and performance.
To address this problem, an adhesion promotor has been applied before the photoresist to limit its curling. In MOS processing, an adhesion promoter such as hexamethyldisilazane (HMDS) can applied to an oxide layer surface of the wafer before it is coated by a photoresist. The adhesion promoter can be applied by any one of several common coating techniques. For example, a quantity of the HMDS can be coated onto the wafer and the wafer then can be spun at speeds of from 3000 to 6000 revolutions per minute (rpm) to spread the adhesive by centrifugal force. Alternatively, a desired thickness of coating can be attained either by immersing the wafer in a solution of adhesion promoter, or by depositing vaporized adhesion promoter onto the wafer. Deposition of photoresist can then proceed as usual.
However, the use of adhesion promoters has some inherent disadvantages. One disadvantage is that many adhesion promoters--including HMDS--can introduce contaminants, which can adversely effect the photoresist. Another disadvantage is that thin adhesion-promoting films on a wafer can lead to interference of the ultraviolet light used to expose the photoresist. This interference makes precise control of opening widths difficult. In addition, because some coatings can chemically react with some adhesives, the method of applying adhesives on the wafer to improve the photolithographic processing is limited in practical application. Toxicity and/or flammability of adhesives makes the use of adhesives inconvenient and hazardous, since great attention and care must be paid during movement and application.
What is needed is a photolithographic process that improves photolithographic precision without the use of adhesion promoters, and without introducing toxics, contaminants, or side reactions. This process should also be applicable to various wafer and layer compositions, and with various coating materials.